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governor to give a higher maximum engine speed.
8. The water flow sensing unit opens only when the
correct pressure difference is obtained between
compressor delivery air pressure and water
pressure. The system is brought into operation when
the engine throttle lever is moved to the take-off
position, causing microswitches to operate and
select the air supply for the turbine pump.
9. The sensing unit also forms a non-return valve to
prevent air pressure feeding back from the discharge
jets and provides for the operation of an indicator
light to show when water is flowing.
Water injection
185
Rolls-Royce Pegasus
Rolls-Royce RB 108
The RB108 was the first engine to be
designed specifically as a direct VTOL engine.
First running in July 1955 the engine was subsequently
thrust rated at 2340 lb, giving a
thrust to weight ratio of 8.7:1. In addition to
powering a variety of VTOL test rigs, the
RB108 flew in a Gloster Meteor, the Short
SC1 and the Marcel Dassault Balzac.
18: Vertical/short take-off
and landing
Contents Page
Introduction 187
Methods of providing
powered lift 189
Lift/propulsion engines
Lift engines
Remote lift systems
Swivelling engines
Bleed air for STOL
Lift thrust augmentation 194
Special engine ratings
Lift burning systems
Ejectors
Aircraft control 197
Reaction controls
Differential engine throttling
Automatic control systems
INTRODUCTION
1. Vertical take-off and landing (VTOL) or short
take-off and landing (STOL) are desirable characteristics
for any type of aircraft, provided that the normal
flight performance characteristics, including
payload/range, are not unreasonably impaired. Until
the introduction of the gas turbine engine, with its
high power/weight ratio, the only powered lift system
capable of VTOL was the low disc loading rotor, as
on the helicopter.
2. Early in 1941, the late Dr A. A. Griffiths, the then
Chief Scientist at Rolls-Royce, envisaged the use of
the jet engine as a powered lift system. However, it
was not until 1947 that a light weight jet engine,
designed by Rolls-Royce for missile propulsion,
existed and had a high enough thrust/weight ratio for
the first pure lift-jet engine to be developed from it.
3. In 1956 the Bristol Aero-Engine Company was
approached by Monsieur Michel Wibault with a
proposal to use a turbo-shaft engine and a reduction
gearbox to drive four centrifugal compressors which
would be situated two on each side of the aircraft.
The casing of these compressors could be rotated to
change direction of the thrust (fig. 18-1). The concept
incorporated two original ideas i.e. the ability to
deflect the thrust over the complete range of angles
from the position for normal flight to that for vertical
lift and a system where the resultant thrust always
acted near to the centre of gravity of the aircraft.
187
Vertical/short take-off and landing
188
4. The principle proposed by M. Wibault was
developed by using a pure jet engine with a free
power turbine to drive an axial flow fan which
exhausted into a pair of swivelling nozzles, one on
each side of the aircraft. A further development was
to use the fan to supercharge the engine, exhausting
the by-pass air through one pair of swivelling nozzles
and adding a second pair of swivelling nozzles to the
Fig. 18-2 Lift/Propulsion engine.
Fig. 18-1 Michel Wibault’s ground attack gyropter (concept) 1956
exhaust system from the engine turbine. In this way
the first ducted fan lift/propulsion engine (the
Pegasus) evolved (fig. 18-2).
5. Subsequent experience with the Pegasus engine
in the Harrier V/STOL fighter aircraft (fig. 18-3), lead
to the development of the short take-off and vertical
landing (STOVL) operational technique. In this way
the additional lift generated by the aircraft wing, even
after a short take-off run, provided a large increase in
the payload/range capability of the aircraft compared
to a pure vertical take-off. Vertical landing had
several operational advantages compared to a short
landing and so was maintained.
METHODS OF PROVIDING POWERED LIFT
6. Although the Pegasus engine is the only V/STOL
engine in operational service in the Western World
there are several possible methods of providing
powered lift, such as;
(1) Deflecting (or vectoring) the exhaust gases
and hence the thrust of the engine.
(2) Using specially designed engines for lift only.
(3) Driving a lift system, which is remote from the
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